Most industrial and semiconductor vacuum processes are performed in high vacuum environments, where pressures are between 10.sup.-4 to 10.sup.-8 torr. Ultra-high vacuums, where pressures are below 10.sup.-8 torr, often use the technique called "Getter Pumping". The present invention has advanced the use of the Getter pumping technique into high-vacuum applications, with significant performance-advantages over conventional pumping techniques.
Gettering is a process that pumps or purifies gases by chemical reaction with an active metal material. For example, oxygen is pumped by forming a metallic oxide. Getter material will not pump inert gases such as helium, argon, krypton, neon or xenon. Evaporable getters are presently in use in the vacuum industry for pumping. In this case, the getter material, usually titanium, is deposited by evaporation onto a substrate, such as the lid of a vacuum chamber, as a thin film. The thin film is quickly used up by reaction, and must be constantly replaced, in the pressure range of between 10.sup.-3 to 10.sup.-7 torr. In fact, it is difficult to replace the film fast enough to use in this pressure range, so this process is generally only used at ultrahigh vacuum (below 10.sup.7 torr), where there are fewer molecules requiring pumping.
Non-evaporable getters are materials that can be used by heating a solid material to temperatures high enough to make it react with the active gases to be pumped. In general, the gases will react on the surface of the getter material, and then slowly migrate into the bulk or body of the getter material. The temperature of a given getter material will control both the rate of reaction and the speed at which the migration occurs. A new charge of getter will be covered with reacted material and will require "activation" before it will pump efficiently. In most cases, this requires a short heating cycle that is at a higher temperature than the operating temperature. The pumping speed of the pump is dependent upon the amount of getter material available for reaction, and the amount of surface area available for reaction.
As stated above, the use of getter-materials as vacuum-pumping vehicles is well-known. The "gettering" process has been around since the early days of the electron tube industry. Materials that react with chemically-active gases to produce low, vapor-pressure compounds were placed in electron tubes to "get" the gases. The term has survived, as did the terms "getters" and "gettering".
Getter-materials are used in many products and processes where one needs to maintain a vacuum against small gas-loads. Sputter-ion pumps and titanium sublimation-pumps use getter-materials in their operation. Strip-mounted films of getter-material also are used as hydrogen pumps in accelerators. The common thread that runs through these gettering systems is that each is a relatively low-throughput device that is ideally suited for clean, ultra-high vacuum processors, where gas-loads are low.
As stated above, getter-pumps may be divided into two basic types: Deposited film (evaporable) or stable state (non-evaporable). Deposited-film, evaporable getters are the more common of the two. As explained above, thin films of getter-material are deposited on host surfaces, such as a chamber wall, where the gettering action takes place. These surfaces are at room temperature, or are cooled below room temperature. The deposited (getter) films are formed by sputtering, as in sputter-ion pumps, or by evaporation, as in sublimation pumps. Titanium is the most commonly deposited getter material. Unfortunately, these films are quickly used up by reaction with the pumped active gases, and must be continually renewed. This means that the gas-load the film is expected to pump is proportional to the rate of renewal needed for the film material. Although deposited films are perfectly capable of pumping a system down to high vacuum from roughing pressure, they have difficulty in meeting a steady gas-load at these pressures. These films also have lower pumping speeds at conventional roughing pressures, so they are difficult and time-consuming to use during the first part of a pump-down cycle. Deposited films do not reach their best performance-levels until ultra-high vacuum (UHV) levels are reached. In addition, they require a significant surface area upon which to be deposited. Although deposited-film getters are clean, their low throughput in high vacuum is compounded by their need for large host surfaces. This usually results in peeling and powdering of the exhausted films, necessitating frequent cleaning of the pump.
The second type, steady-state, or non-evaporable, stable getter pumps use the same pumping mechanism as the thin-film getters, in that they react with the active gases to be pumped. But their similarity stops there. Steady-state getter material remain as solid forms that continually sorb the gases. Normally, these materials are heated during operation. Heat helps diffuse the pumped, active gases into the bulk of the getter-material, which then continually exposes fresh getter-material surfaces. Steady-state getter-materials are commercially available in strip form, where a getter-film is bonded to a support strip, or in bulk forms, such as pills, pellets, or chunks. As in all gettering systems, a solid-state getter-material has a finite ability to sorb gases before it becomes saturated.
As stated before, most commercial vacuum processes only require high vacuum, not ultra-high vacuum, and, therefore have not used the getter-pumping method, because of the disadvantages summarized above. These high-vacuum (HV) commercial processes require short pump-down times and repeated pump-down cycles between process-loads. In many processes, however, fast pump-down is not enough. Cleanliness of the pumping process also is vital, as more stringent processes are developed.
Conventional, non-gettering, pumping techniques typically employ an oil-sealed mechanical pump, which cannot easily reach roughing pressures below several millitorr. The amount of gas that any getter-pumping system would have to sorb at these relatively-high pressures would exhaust its ability to pump, if it had to pump down repeatedly from these high pressures. An oil-sealed, mechanical pump requires an additional high-vacuum pump, such as a turbomolecular pump, to reach lower pressures before any gettering pump could be employed. Molecular-drag pump technology changed this by not only providing cleaner roughing, but by also allowing roughing pressures of 10.sup.-4 to 10.sup.-6 torr to be routinely achieved. Roughing to these pressures has opened a new application for getter pumping. Experiments with a small, strip getter-pump demonstrated that one can easily and quickly evacuate a chamber when roughing pressures are reduced below 10.sup.-4 torr. However, there is a problem with using a molecular-drag pump/getter strip pump system for industrial processes. When the pump has to re-evacuate the chamber shortly after it has been opened to air between process cycles, the throughput becomes limited. This limitation is traceable to the amount of surface area of getter material available for pumping. When the pump meets a steady-gas load, the getter surface becomes covered with reacted gas, which diffuses into the bulk of the material at a rate governed by its composition and temperature. If the surfaces are in equilibrium with a small gas load, the surface will not be able to recover its full pumping speed quickly enough to deal with a higher gas load when the system is opened to air and roughed down quickly.